In most known electrophotographic copy machines, an image of the object being copied is directed to an image carrier. The image can be of the entire document (a flash exposure system) or of only a portion of the document (a scanning system). In the latter case, an image of the document is built up in time as the object being copied is scanned.
Scanning systems have been embodied in plain and coated paper copiers. In the former type, the image is electrically recorded on an intermediate carrier from whence it is transferred to the copy paper. In the latter type, the image is directly laid down on the coated paper. Accordingly, the term image carrier will be applied to either the intermediate carrier (for plain paper copiers) or the copy paper (for coated paper copiers).
Various document copier machines have been produced with the capability of reducing the size of copies made from an original, usually placed on a transparent document support. Most of these machines, however, have been designed for providing specific discrete reduction ratios, e.g., 0.75:1 or 0.6:1. Rarely has an attempt been made to provide a document copier with the capability of continuously variable reduction from ratios such as 1:1 to another ratio such as, for example, 0.647:1. The few attempts that do appear in the prior art, for example. U.S. Pat. Nos. 2,927,503 (Zollinger) and 3,395,610 (Evans) have operated with a flash exposure system. Flash exposure systems have the disadvantage of requiring a flat imaging surface which thus cannot use the rotating image carrier, or drum, found in the most popular commercially available copying machines. The requirement of a flat image plane also requires a mechanically more complex machine which occupies more space than does a machine which employs the rotating drum. Other disadvantages of the flash system are a higher power requirement and a machine which can temporarily blind an operator if the flash is eye-observed. Despite these disadvantages, most prior variable reduction systems opt for the flash exposure procedure to take advantage of the simplicity of its concept. For example, one of the complexities of the scanning system in a reduction copy machine is a requirement that the velocity of the scanning carriage relative to the surface velocity of the image carrier be changed as the reduction ratio is changed. However, systems capable of this function exist in the prior art, for example, U.S. Pat. Nos. 3,614,222; 3,897,148; and 3,542,467, but these systems are limited to two, three and five discrete reduction ratios, respectively, and therefore, only require two, three or five velocity ratios.
Since the carriage cannot be accelerated instantaneously, each carriage movement may include six phases. A scan movement is accomplished from a start of scan position to a home position encompassing acceleration, constant velocity and deceleration phases. Image transfer is accomplished during the constant velocity phase. This phase is only nominally at constant velocity since the objective is to maintain constant the relationship between carriage velocity and image carrier velocity. The carriage must also be moved from the home position to the start of scan position (termed rescan) which movement may also encompass three similar phases of motion.
In addition to selecting scan velocity, of the "constant" velocity phase of movement, in a manner correlated with a selected reduction ratio, the length of the scan must also be selected. For example, at 1:1, an 11 inch document is scanned into an 11 inch image, but at 0.647 reduction, a 17 inch document is scanned into the same 11 inch area. Thus, not only must the scan velocity be appropriately selected, but also the length of the scan. Of course, of primary concern is the length of the "constant" velocity phase. However, different velocities require different travel lengths for acceleration and deceleration as well.
In addition to selection of scan velocity and length, the relative position of the leading edge must also be located. Desirably, the leading edge of the copy paper must be matched to the leading edge of the image area. Therefore, if both the document and the copy paper are 81/2.times.11, it is necessary to place the leading edge of the image at the leading edge of the image area in order to transfer the entire image to the copy paper. If a document of 17 inch size is placed on a document support, it must still be squeezed into an 11 inch image area for transfer to an 81/2.times.11 inch sheet of copy paper. Therefore, unless over reduction is practiced, the leading edge of the image of the reduced document must also fall on the leading edge of the image area. Furthermore, in a scanning system, the carriage must scan the document at a velocity selected in dependence on the desired reduction. The different velocities require different acceleration times (and distances) and thus, the position of the carriage at the beginning of its movement and the time the movement commences relative to the image area on the drum must also be properly selected. Accordingly, the scanning carriage position at the beginning of movement must be selected in terms of time or space (or both) so that the carriage begins to scan the document at the same position relative to the image carrier surface, regardless of reduction.
The referred to patent application discloses a scanning, continuously variable reduction photocopier which meets the heretofore stated objectives of altering scan velocity, scan length and starting document scan location. In the referred to patent application, however, the scanning carriage drive system is mechanically coupled to the main drive motor and various cams, lead screws and drive bands are selectively positioned, rotated or adjusted so as to produce the desired motion. However, we believed it desirable to provide a system meeting these objects in which the scanning carriage drive was electrically controlled, rather than being mechanically coupled to the main drive motor. By providing an electrical control for the scanning carriage drive a number of advantages are derived. For one thing, the main drive motor can be reduced in size and the previously noted mechanical linkages can be eliminated, thus significantly reducing the total required machine drive torque, total machine volume and significantly reducing mechanical part count. In one embodiment, this torque reduction was over 30%, machine volume decreased by 500 in.sup.3 and the mechanical drive part count was reduced from over 300 to about 60.
Since the carriage drive is no longer mechanically coupled to the main drum motor, flexibility is provided in selecting the relative timing of various machine cycles and this allows a reduction in the "time to first copy" which is an important characteristic of copying machines.
In addition, decoupling carriage drive and main motor drive enables carriage motion to adapt for different paper lengths maintaining a common image reference to both simplify gating paper feed and eliminating detack marks from within the image area since the paper detack is common for all lengths of paper. Furthermore, mechanical couplings to the optical elements (lens and carriage positioning) can be eliminated significantly simplifying the optical controls. The overall simplification of the machine, reduction in number of components and machine volume, and decrease in power requirements will lead to a significant reduction in required maintenance.